Often it just comes from how comfortable the player is in passing with his left foot (as quite often the centre back would be making a pass up the line to the fullback).
Personally I think the importance of that is often exaggerated on this board - I think you can still play the vast majority of passes from that position with your right foot. Ala Harry Maguire, for example. And I've not noticed Toby or Tanganga looking particularly one-footed, ala Lamela or someone. But both of those claims are quite hard to give evidence for, so I might just leave it there!
If it really is the case that only Rodon or Dier can play on the left, I'd go Tanganga/Dier in EL and Toby/Rodon in PL.